Colorectal cancer is the second most common cause of cancer-related mortality in Europe and North America. This cancer affects nearly 150,000 patients and results in more than 60,000 deaths in the United States per year. Despite significant advances in the management of the colon cancer patient, there has been little change in survival rates over the past 50 years. The primary cause of death relates to the development of distant metastases to organs such as the liver and lungs. Unfortunately, colon cancer still remains one of the most common types of epithelial malignancies in both genders and is essentially incurable when it reaches the most advanced stages.
Surgical resection remains the present standard of care for patients with localized colorectal cancer. Several adjuvant chemotherapy strategies have also emerged and the use of 5-fluorouracil (5-FU) with leucovorin (LV) rescue is now established in stage III colon cancer. Considering the high re-occurrence rates of colon cancer and the side-effects of surgical and chemical therapies, the discovery of novel compounds that block, reverse, delay or prevent the development of invasive large bowel neoplasms would be of major importance.
Adenocarcinoma accounts for 90-95% of all colorectal cancer and the majority of the human cultured cell lines reflect this phenotype. Table 1 summarizes the differences in some available human colon cell lines in relation to age, gender, histology/grade, and source (e.g., ascites vs. primary tumor). These cultured cell lines provide a rich opportunity to evaluate novel compounds for efficacy and to establish their mechanism of action.
TABLE 1NCI cultured human colon cancer cell lines.Gen-PatientTreat-Cell lineaderAgeHistologymentSourceCOLO 205M70AdenocarcinomaYAscitesHCC-2998CarcinomaNHCT-15AdenocarcinomaHCT-116Adenocarcinoma/grade IIIHT29F44AdenocarcinomaPrimaryKM12AdenocarcinomaSW-60M51AdenocarcinomaNMetastasisaAvailable cell lines from the National Cancer Institute (NCI).
Tubulin, the subunit protein of cellular microtubules, is the target of several effective cancer chemotherapeutic agents currently in clinical use. Tubulin is composed of an α/β heterodimer, and at least six human α-tubulin and seven human β-tubulin isotypes (gene products) are known. Overall, the repertoire of β-tubulin isotypes is believed to play a significant role in development and the building of specialized microtubule-based cellular structures, and general disruption of cellular microtubules is one target for cancer chemotherapy that has proven to be effective.
In many organisms, both α and β tubulin isotypes differ by their tissue distributions. In mammals, the βI and βIV isotypes are quite widespread, and βII is less so, while βIII and βVI have narrow distributions and βV distribution is unknown. As a tool for localizing the isotypes, the preparation of monoclonal antibodies specific for βI, βIII, βIV and βV isotypes have been reported (Banerjee et al., J. Biol. Chem. 1988, 263:3029-3034). β-isotypes have been localized in several human tissues including oviduct, skin, colon, and pancreas with striking differences in their tissue distributions. In fact, there is little or no βIII in these tissues, except for the columnar epithelial cells of the colon (Roach et al., Cell Motility and the Cytoskeleton 1998, 39:4:273-285).
Normal cellular architecture, growth, division, and intracellular transport are dependent on microtubules. Microtubules are versatile and highly dynamic structures that undergo rapid changes in response to cellular signaling from a variety of stimuli. The dynamic instability of microtubules is critical for their normal functions. Drugs that disrupt the dynamic response of microtubules can lead to altered microtubule function, abnormal cellular metabolism, and can ultimately lead to apoptosis.
In cell lines resistant to microtubule-stabilizing drugs that express heterozygous tubulin mutations, the relative amount of mutant tubulin expression is important. In these cell lines, the absence of βII- and βIVa-tubulin has been demonstrated, and an increased level of expression of βIII-tubulin in resistant cells has been confirmed (Verdier-Pinard et al., Biochemistry, 42(18):5349-57, 2003), indicating that this tubulin isotype may have a significant role in taxol resistance.
Antimicrotubule agents comprise some of the most widely used and effective cancer chemotherapeutic agents in clinical use (Rowinsky, E. K. and Tolcher, A. W. Antimicrotubule Agents. In: V. T. J. DeVita, S. Hellman, and S. A. Rosenberg (eds.), Cancer Principles and Practice of Oncology, 6th edition, Vol. 1, pp. 431-447. Philadelphia, Pa.: Lippincott Williams and Wilkins, 2001). Prompted by the clinical successes of the vinca alkaloids and taxanes, significant efforts have been focused on identifying new agents that have a similar mechanism of action, but superior properties including the ability to circumvent drug resistance mechanisms, exhibit better solubility and oral availability.
A serious problem associated with the treatment of cancer is the development of drug resistance. Some tumors are intrinsically resistant to chemotherapy and others develop drug resistance during chemotherapy. A significant proportion of tumors are multidrug resistant because of overexpression of membrane proteins that act as drug efflux pumps. Overexpression of the MDR-1 gene product, P-glycoprotein (Pgp), leads to diminished intracellular drug accumulation and to attenuated cytotoxic effects. Clinically, multidrug resistance imparted by the expression of Pgp can limit the utility of many currently available agents including vinblastine, vincristine, taxol and docetaxol. There is a need for new drugs that can circumvent multidrug resistance.
A second major reason for the development of new microtubule-active agents is that microtubule disruptors are in some cases effective against tumors that express abnormal p53. The tumor suppressor gene encoding p53 is the most frequently mutated gene in human cancers. It is estimated that half of all cancers in the United States exhibit altered p53 (Hollstein et al. Science, 253: 49-53, 1991).
In addition, compounds that target cellular microtubules have recently been found to exhibit antiangiogenic activities and this may contribute to their antitumor and anticancer efficacies (Miller, et al.,. J. Clin. Oncol. 19, 1195-1206, 2001). The taxanes, taxol and docetaxel, vinblastine, vincristine, combretastatin (Holwell et al., Anticancer Research. 22(6C):3933-40, 2002) and 2-methoxyestradiol all have antiangiogenic activity in vivo (Miller, et al., J. Clin. Oncol., 19:1195-1206, 2001).
Angiogenesis is the process by which new blood vessels are formed from pre-existing blood vessels. This process is complex and begins with the degradation of the basement membrane by proteases secreted by activated endothelial cells. Migration and proliferation leads to the formation of solid endothelial cell sprouts into the stromal space. Vascular loops and capillary tubes develop with formation of tight junctions and deposition of new basement membrane. This process is important in normal reproduction, embryonic development, and wound healing. However, improperly regulated angiogenesis has been implicated in many diseases including cancer.
Tumor growth requires the formation of new blood vessels, (i.e., angiogenesis). It is believed that tumor cells initiate and maintain angiogenesis by expressing a network of angiogenic factors, including endothelial growth factors such as vascular endothelial growth factor (VEGF), angiogenic cytokines such as interleukin-8 (IL-8), matrix metalloproteinases (MMP) such as MMP-2 and MMP-9, and adhesion molecules such as integrins and cadherins. Considering the relevance of angiogenesis in tumor progression, anti-angiogenic therapies have emerged as a potentially promising modality of cancer therapy. A variety of purely anti-angiogenic strategies have been developed, including: 1) endogenous angiogenesis inhibitors (e.g., endostatin); 2) blockers of endothelial survival and growth factors/receptors (e.g., VEGF antibody and VEGF receptor tyrosine kinase inhibitor SU6668); and 3) inhibitors of adhesion molecules or MMPs (e.g., antibodies against integrin). Unfortunately, the use of anti-angiogenic agents to treat cancer has proved challenging and purely anti-angiogenic strategies have failed in the clinic. While these agents inhibit tumor angiogenesis in animal studies, complete suppression of angiogenesis or tumor shrinkage in patients has been uncommon.
There is a long felt need in the art for a better method to identify and prepare compounds capable of regulating cancer cells, angiogenesis, endothelial cells, and tumor formation.